High-directivity microstrip coupler having periodically indented conductors

ABSTRACT

A directional coupler employing a microstrip line with a pair of conductors printed on a dielectric block, which has a ground plane adhered to the opposite surface. The inner confronted edges of the pair of conductors are periodically indented with the indentations being staggered so that the spacing between the edges of the conductors remains uniform.

ite

inn tet Inventor Allen ll Podelll Cambridge, Mann.

Appl. No. 4%,2117

Filed .lliine d, W70

Patented llllec. 211, 1197i Assignee Adelina-Rammed (20., line.

Wnlthnm, Maria.

lllllGlll-DlMECTlli/H'lll/ MllClltUS'lllltlllP tCUlUllUEllt HAVING [PEWIUDHCALILY HNlDlENTElD CONlDIlJC'llUlltfi 7 Claims, 7 Drawing ll i gs.

US. Cl 333/110, 333/84 M llnt.Cll llltlllp 5/141 333/10 ll ield all" Search...

[56] lllleiereneen Cited UNlTED STATES PATENTS 3.332.039 7/!967 Oh 333/l0 3,516,024 6/1970 Lange 333/10 Primary Examiner- Herman lltarl Snalbnch Assistant Examiner- Paul L. Gensler Attorney-lienway, .Ienney iii Hildreth AfiS'lllltA'C'll: A directional coupler employing a microstrip line with a pair of conductors printed on a dielectric block, which has a ground plane adhered to the opposite surface. The inner confronted edges of the pair of conductors are periodically indented with the indentations being staggered so that the spacing between the edges of the conductors remains uniform.

PATENTED UEEZ] 19?:

SHEET 1 [IF 2 HNVENTOR ALLEN F. PODELL ATTORNEYS PATENTEU UEBZT 197! $629,733

sum 2 BF 2 COUPLING ISOLATION\ F (GH INVENTOR ALLEN F. PODELL BY W,;Lw

ATTORNEYS I-IIGH-DIRIECTIVITY MICROSTIRIP COUPLER HAVING PERIODICALLY INDIENTED CONDUCTORS FIELD OF THE INVENTION This invention relates in general to a directional coupler operating in the radiofrequency region and more particularly to a high directivity microstrip coupler operating over relatively broad bands.

BACKGROUND OF THE INVENTION The use of transmission lines as directional couplers is well known in the art and the use of rriicrostrips, in which a pair of transmission lines are fixed in a dielectric medium with respect to a ground plane, as directional couplers is also known. Typically microstrip couplers are loosely coupled and exhibit a directivity characteristic which decreases as frequency increases. The dependence of directivity upon frequency appears to result from the fact that in the geometry of a microstrip coupler, the effective dielectric for a wave traveling in the even, or push-push mode, differs from the efiective dielectric for a wave traveling in the odd, or push-pull, mode. This arises from the fact that the even mode wave is traveling through the solid dielectric which lies between the ground plane and the conductors, while the odd mode wave travels partly in the dielectric material underlying the conductors and partly through air. The result of this difference of dielectric is that there is a difference in the velocity for the even and odd mode waves. Since a signal applied between ground and one end of one of the conductors may be considered as a pair of signals, each of one-half amplitude, one applied in the even mode and the other in the odd mode, then this difference in velocity results in a directivity characteristic which varies with frequency. The conventional technique for overcoming this problem is the use of lumped capacitances. However, the bandwidth, even with such modifications, remains limited. Another problem with modifications of this type is the increasing complexity of construction of the devices, since specific components must be added and it is not sufficient simply to print the conductors on the dielectric. This increased complexity, of course, results in an increased cost.

BRIEF SUMMARY OF THE INVENTION Broadly speaking, the directional coupler for the present invention employs a microstrip line including a pair of conductors separated from a ground plane by a solid dielectric material, such as glass epoxy. The two conductors are printed on one surface of the dielectric, the width of each of the conductors and the spacing between them being selected to provide the appropriate characteristic impedance for the line. The inner confronting edges of each of these conductors are periodically indented, either in an undulating or a sawtoothed fashion. The indentations on the edge of one conductor are staggered with respect to the indentations on the other so that the spacing between the edges of the conductors remains uniform. With this arrangement the mutual inductance between the conductors remains substantially the same as for a pair of nonindented conductors, however, the capacitance is increased. The conductors are indented such that the increased capacitance offsets the increase in wave velocity due to the air dielectric and thus the odd mode, which is the only wave that is affected by the indentations, is slowed down so that it travels the length of the coupler in the same time as the even mode, which is propagated entirely through the solid dielectric, traverses the coupler. High directivity couplers with couplings ranging from 7.8 to 27 db. have been constructed using this technique. By employing a pair of 8 db. couplers in tandem, a more closely coupled (approximately 3 db. directional coupler has been constructed. The couplers described are very economical and reproducible to build, since, once the correct pattern has been established, the conductors may be printed on the dielectric with a mask using conventional photoetching circuit printing techniques.

BRIEF DESCRIPTION OF THE DRAWING In the drawing FIG. I is an illustration in plan view of one preferred embodiment of a high directivity coupler constructed in accordance with the principles of this invention;

FIG. 2 is an illustration in cross sectional view of the coupler of FIG. I taken along the line 2-2 of FIG. 1;

FIG. 3 is an illustration in plan view of a second embodiment of a high directivity coupler constructed in accordance with the principles of this invention;

FIG. 4 is an illustration in graphical form of the coupling characteristics of the coupler of FIG. ll;

FIG. 5 is an illustration in plan view of a third embodiment of high directivity coupler constructed in accordance with the principles of this invention;

FIG. 6 is an illustration in plan view of another embodiment of a high directivity coupler constructed in accordance with the principles of this invention; and

FIG. 7 is an illustration in cross-sectional view of the coupler of FIG. 6 taken along the line 7-7 of FIG. 6.

DESCRIPTION OF PREFERRED EMBODIMENTS With reference now to FIGS. 1 and 2, a microstrip coupler is illustrated. A block ll of solid dielectric material, such as glass epoxy, alumina or glass Teflon (Teflon being a trademark of E. DuPont Company for a poly Tetra fluoro ethene) has adhered to its lowest surface a ground plane I7 and printed on its upper surface a pair of conductors I3 and 15. Each ofthe conductors l3 and 115 have a periodically indented inner edge with a straight outer edge. One end of conductor 13 is connected to a coaxial connector 23, while the opposite end is connected to a coaxial connector 22. Similarly one end of conductor I5 is connected to a coaxial connector 2i, while the opposite end is connected to a fourth coaxial connector 20. Each of the coaxial connectors provide for connection to one of the four ports of the coupler, the ports A, B, C and D being formed between the free ends of each conductor and the ground plane 17. The material of the conductors l3 and 15 is typically gold, 0.001 inch thickness. The dimensions of the width and length will, depend upon the specific impedance characteristics and Wavelength. In general, the indentations must have a period much shorter than the wavelength of the waves to be passed along the coupler. Typical values for a 50 ohm characteristic impedance line are an average width of the conductor of 0.020 inch and, for a coupling of 15 or 20 db. a 0.020 inch spacing between the indented inner edges of the two conductors. In FIG. 4 there is illustrated in graphical form a coupling characteristic for the directional coupler illustrated in FIGS. 1 and 2. As can be seen in that illustration, for a coupling of approximately 15 db. over a frequency range of l to 2 GI-Iz., isolation ofapproximately 40 db. is achievable.

Thus an input signal applied at port A between conductor 15 and ground is isolated by at least 40 db. from port D, which appears between one end of conductor I3 and ground, port A being coupled to port B with about 15 db. the output signals at ports B and C being exactly out of phase with each other.

In FIG. 3 there is illustrated a pattern for a pair of conductors similar to those illustrated in FIG. 1. In the embodiment of FIG. 3, however, each of the conductors 25 and 26 has a periodically indented edge which is undulating rather than sawtoothed. Similar results to those obtainable with the embodiment of FIG. I are achievable.

In FIG. 5 there is illustrated a tandem arrangement of high directivity couplers, each of the individual couplers being an 8 db. coupler, the resultant overall coupling then being approximately 3 db. over an octave. In the configuration illustrated, one conductor 30 commences at input port A and is a single printed conductor which terminates at end 31. At this point a crossover connection is made to conductor 33. The crossover connection may be a welded strip, a bond wire or beam lead. In similar fashion, conductor 34 commences at isolated port D and at the end of this conductor 35 a crossover connection 38 provides an electrical path to the remaining conductor 39. A coupler has been constructed according to the plan shown in FIG. 5 using 0.001 inch gold thickness for the conductors of 0.020 inch alumina dielectric, with a 0.003 inch gap between the confronting indented edges of the conductors. The crossovers 37 and 38 were made with a pair of 0.001 inch bond wires. With this configuration 3 db. coupling was achieved to the output ports B and C with 27 db. minimum isolation to the isolated port D.

In FIGS. 6 and 7 there is illustrated in a different configuration, a directional coupler in which the slab of solid dielectric 45 has a pair of ground planes 48 and 49 at either side with the two signal-carrying conductors 50 and 51 being on opposite sides of the slab of dielectric material 45. In the cross-sectional view of FIG. 7, the thickness of the conductors has been exaggerated in order to illustrate the construction. It is the odd mode wave traveling between conductors 50 and 51 which sees an entirely dielectric medium in this configuration while the even mode between each of the conductors and the ground plane travels through by a dielectric medium that is partially the solid medium and partially air. The periodically indented edges of the conductors 50 and 51 are now made to provide the compensating extra capacity with the ground planes 49 and 48 as illustrated. In this arrangement the indented edges of the center conductors 50 and 51 must be such that they do not vary the capacitive coupling between these two conductors since that would again insert imbalance between the wave velocity of the two modes.

While the invention has been described in terms of specific dielectric materials and particular dimensions, it will be under'stood that the variety of dimensions may be employed and the variety of dielectric materials may also be used. Again, while the invention has been described in terms of a solid dielectric and air it will be apparent that two different valued dielectric materials might be employed and the same techniques utilized.

What is claimed is:

l. A high directivity coupler for radiofrequency waves comprising:

a block of solid dielectric material;

first and second substantially identical elongated electrical conductors located in parallel juxtaposition on a first surface of said block of dielectric material, the confronting edges of said conductors being periodically indented, the indentations of said first conductor being staggered with respect to the indentations of said second conductor whereby the spacing between the edges of said first and second conductors is maintained substantially constant over their length, the outer edges of said first and second conductors having a significantly shorter length than said periodically indented confronted edges;

a third conductor positioned on the surface of said dielectric block opposite to said first surface, said third conductor being coextensive with said first and second conductors, one of said conductors being connected to a point of potential reference, the ports of said directional coupler being formed between said point of potential reference and each of the ends of the two conductors not connected to said point of potential'reference.

2. A coupler in accordance with claim 1 wherein said third conductor is connected to said point of potential reference.

3. A directional coupler in accordance with claim 2 wherein the outer edges of said first and second conductors are straight and substantially parallel to one another.

4. A coupler in accordance with claim 2 wherein said dielectric block is formed of glass epoxy and said first and second electrical conductors are formed of gold.

5. A directional coupler in accordance with claim 2 wherein said periodically indented edges of said first and second conductors are undulating.

6. A directional coupler in accordance with claim 2 wherein said periodically indented edges of said first and second conductors have a sawtooth configuration.

7. A four port directional coupler for radiofrequency waves comprising:

a block of dielectric material;

a conducting electrical plane disposed on first surface of said block of dielectric material, said plane being con nected to a point of potential reference;

a first pair of inner and outer positioned electrical conductors disposed on a second surface of said dielectric block opposite to said first surface, said first pair of electrical conductors being elongated along a first axis and positioned in parallel juxtaposition to one another, the outer edges of said first pair of conductors being substantially straight and the confronting edges being periodically indented, the indentations being staggered such that the spacing between said inner edges is substantially constant; 1

a second pair of electrical conductors, substantially identical in shape and dimension to said first pair of electrical conductors, said second pair of electrical conductors being disposed along said first axis and in line with said first pair of electrical conductors;

a third pair of electrical conductors substantially identical in shape and dimension to said first pair of electrical conductors, said third pair of electrical conductors being positioned on said second surface of said dielectric block along second axis parallel to and laterally displaced from said first pair of electrical conductors;

a fourth pair of electrical conductors substantially identical in shape and dimension to said first pair of electrical conductors, said fourth pair of electrical conductors being disposed on said second surface of said dielectric block along said second axis and in line with said third pair of electrical conductors; the outer positioned conductor of said first pair of conductors being electrically connected to the inner positioned one of said second pair of conductors, the inner positioned one of said second pair of electrical conductors being electrically connected to the inner positioned one of said fourth pair of electrical conductors, which is electrically connected to the outer positioned one of said third pair of electrical conductors;

the outer positioned one of said second pair of electrical conductors being electrically connected to the inner positioned one of said first pair of electrical conductors, which is in turn electrically connected through the inner positioned one of said third pair of electrical conductors to the outer positioned one of said fourth pair of electrical conductors, the free ends of the outer ones of said first, second, third and fourth pair of electrical conductors serving as the four ports of said directional coupler. 

1. A high directivity coupler for radiofrequency waves comprising: a block of solid dielectric material; first and second substantially identical elongated electrical conductors located in parallel juxtaposition on a first surface of said block of dielectric material, the confronting edges of said conductors being periodically indented, the indentations of said first conductor being staggered with respect to the indentations of said second conductor whereby the spacing between the edges of said first and second conductors is maintained substantially constant over their length, the outer edges of said first and second conductors having a significantly shorter length than said periodically indented confronted edges; a third conductor positioned on the surface of said dielectric block opposite to said first suRface, said third conductor being coextensive with said first and second conductors, one of said conductors being connected to a point of potential reference, the ports of said directional coupler being formed between said point of potential reference and each of the ends of the two conductors not connected to said point of potential reference.
 2. A coupler in accordance with claim 1 wherein said third conductor is connected to said point of potential reference.
 3. A directional coupler in accordance with claim 2 wherein the outer edges of said first and second conductors are straight and substantially parallel to one another.
 4. A coupler in accordance with claim 2 wherein said dielectric block is formed of glass epoxy and said first and second electrical conductors are formed of gold.
 5. A directional coupler in accordance with claim 2 wherein said periodically indented edges of said first and second conductors are undulating.
 6. A directional coupler in accordance with claim 2 wherein said periodically indented edges of said first and second conductors have a sawtooth configuration.
 7. A four port directional coupler for radiofrequency waves comprising: a block of dielectric material; a conducting electrical plane disposed on first surface of said block of dielectric material, said plane being connected to a point of potential reference; a first pair of inner and outer positioned electrical conductors disposed on a second surface of said dielectric block opposite to said first surface, said first pair of electrical conductors being elongated along a first axis and positioned in parallel juxtaposition to one another, the outer edges of said first pair of conductors being substantially straight and the confronting edges being periodically indented, the indentations being staggered such that the spacing between said inner edges is substantially constant; a second pair of electrical conductors, substantially identical in shape and dimension to said first pair of electrical conductors, said second pair of electrical conductors being disposed along said first axis and in line with said first pair of electrical conductors; a third pair of electrical conductors substantially identical in shape and dimension to said first pair of electrical conductors, said third pair of electrical conductors being positioned on said second surface of said dielectric block along second axis parallel to and laterally displaced from said first pair of electrical conductors; a fourth pair of electrical conductors substantially identical in shape and dimension to said first pair of electrical conductors, said fourth pair of electrical conductors being disposed on said second surface of said dielectric block along said second axis and in line with said third pair of electrical conductors; the outer positioned conductor of said first pair of conductors being electrically connected to the inner positioned one of said second pair of conductors, the inner positioned one of said second pair of electrical conductors being electrically connected to the inner positioned one of said fourth pair of electrical conductors, which is electrically connected to the outer positioned one of said third pair of electrical conductors; the outer positioned one of said second pair of electrical conductors being electrically connected to the inner positioned one of said first pair of electrical conductors, which is in turn electrically connected through the inner positioned one of said third pair of electrical conductors to the outer positioned one of said fourth pair of electrical conductors, the free ends of the outer ones of said first, second, third and fourth pair of electrical conductors serving as the four ports of said directional coupler. 